1. PDF dokument

NEPORUŠITVENO PREIZKUŠANJE
Detection of delaminations and porosity in glass fiber reinforced polymer
composite with ultrasonic method
Zoran BERGANT, Joseph JANEZ, Janez GRUM
Abstract: In this paper, the pulse-echo ultrasonic C-scan method was used for the examination of various
processing defects in a composite plate produced by vacuum infusion (VI). Several artificial defects were
embedded into the test plate, varying in shape, volume and depth. During the specimen preparation with VI,
air was introduced through a small non-sealed spot in the vacuum membrane. In the selected location, the
PVC and aluminium foils and aluminium chips were inserted between layers. The immersion ultrasonic system
with water couplant was used to study the defects in the plate with the 4D C-scan method using different
frequencies and gate settings to find the optimum set of parameters to detect defects. The C-scan images
show that the detection and location of PVC foils and aluminium chips were successful. Also, porosity can be
clearly detected for individual layers. The thin, 0.04 mm aluminium foils were not detected with this method.
Key words: Ultrasound examination, C-scan, GFRP, epoxy, composite, vacuum infusion
■■ 1 Introduction
Composite materials consist of two
or more elements, one of which, the
fibre, is dispersed in a continuous
matrix phase [1]. The two elements
work together to produce material
properties that are different to the
properties of the elements on their
own. Composites offer the designer
a combination of properties not
available in traditional materials.
The production process for polymer
matrix composites has the potential
to introduce a variety of processing
defects. Fibre misalignment can
occur when fibres are laid up. Fibres
in the same layer may be misaligned
Dr. Zoran Bergant, univ. dipl. inž.,
University of Ljubljana, Faculty
of Mechanical Engineering;
Joseph Janez, univ. dipl. inž., Mistras Group, SA, France;
prof. dr. Janez Grum, univ. dipl.
inž., University of Ljubljana, Faculty of Mechanical Engineering
40
relative to each other or misalignment between layers might occur.
Foreign inclusions such as dirt and
debris may contaminate the matrix
or act as a local stress concentration in the finished product. These
defects can lead to delamination either during the production process
or later on when the component is
in service.
The use of composite materials is
constantly increasing in aviation,
aerospace and energy (wind turbines) where reliable inspection methods must be applied frequently
to ensure safe operation. There are
many detection principles which are
continuously evolving. Among those, ultrasonic and thermographic
methods are the most successful
and widely used [1]. Composite materials are highly anisotropic with a
high amount of boundaries which
lead to high noise levels which limit
detection accuracy. With improved
transducers and signal processing
methods, modern C-scan methods
have become very effective for
composite testing. C-scan analysis
provides information of the location and size of the impurities, fibre
layup plan and misalignment, delaminations, impact damage, fibre
fatigue damage, water filled cores,
etc. [2-3]. There are also new ultrasonic methods being developed
with guided ultrasonic waves [4].
Hasiotis et al [5] studied different
C-scan techniques, focusing on
two materials: glass fibre reinforced
polymer (GFRP) and carbon fibre reinforced polymer (CRFP). In general,
it is more difficult to detect defects
in glass fibre reinforced composites
due to the high reflectivity of glass
fibres.
Vacuum infusion (VI) is a widely
used technique for the production
of large composite parts [6]. A typical mistake in vacuum infusion is
the non-sealed vacuum membrane
with a large impact on quality and
in most cases a waste of component
and many hours of work. The aim of
our research is to study and detect
the various artificial defects in glass
fibre reinforced polymer composite
using the 4D C-scan technique.
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NEPORUŠITVENO PREIZKUŠANJE
Table 1. Composite properties
Fibre
Epoxy matrix
No. of layers
Thickness [mm]
Density
[kg/m3]
Fibre volume content [%]
Twill weave
biax,
Glass fibres
280 g/m2
Renlam LY
5138/HY 5130
Mix. Ratio:
100:23
6
1.88
1267
54
■■ 2 Experimental setup
2.1 Specimen preparation
For the ultrasonic C-scan inspection, glass fibre reinforced epoxy
specimen was prepared using the
vacuum infusion (VI) process. The
specimen consists of 6 layers of bi-directional twill weave glass fibres
(280 g/m2) impregnated with cold
curing epoxy resin Renlam LY 5138/
HY5130 by Huntsmann. Defects
were introduced by inserting round
pieces of aluminium and PVC foils
between layers.
Figure 1. Vacuum infusion mould lay-up, p1=inlet pressure, p2=outlet pressure [6]
2.2 Vacuum infusion
Vacuum infusion (VI) is a widely used
moulding process for the production of large composite structures. Its
popularity is partly due to the low
cost of the tooling and the environmental safety [6]. In addition, low
operator involvement increases the
repeatability of the process compared to open mould techniques, such
as hand lay-up or spray-up, and the
components are of relatively high
fibre content, up to 60 % of the volume. On the other hand, the complex preparation procedure of VI
could easily lead to defects, such as
air voids, because of not sufficiently
sealed spots. The VI mould lay-up is
shown in Figure 1.
The lay-up consist of 6 layers of 280
g/m2 glass fabric, an inlet connection in the bottom left corner and
outlet connection in the upper right corner, horizontally placed inlet
and outlet resin channels, peel-ply
and permeable infusion medium,
a)
b)
Figure 2. Glass fibres and preparation for vacuum infusion, (a) Glass fibres,
(b) embedded defects: Al and PVC foils
vacuum bag and sealant tape. The
layers were placed on a flat glass surface, coated with the release wax.
Four types of defects were embedded: (1) Al-foil, thickness 0.04 mm,
(2) PVC foil, thickness 0.1 mm, (3)
Al-chips in random locations, (4) air
voids (clusters) in random locations.
The Al- and PVC-foils were cut to
circular shape with dimensions from
Æ 4 to 12 mm with 2 mm increments in size. Two sets were placed
between layers. The first row of defects from Æ 4 to 12 mm was placed
in the middle of the plate thickness,
between the 3rd and 4th layer. The
second row was placed between
the 1-2nd layer, 2-3rd, 3-4th and 5-6th
layer, consequently. The plate was
cured at room temperature.
2.3 Generation and
distribution of air voids
A common processing defect during vacuum infusion is the air leakage through the non-sealed or
damaged vacuum bag. Air was introduced into the plate through the
small damage in the vacuum bag.
The travelling direction of air voids
was the same as the infusion front
direction. After the disconnection
of the air compressor, air voids dispersed throughout the entire plate.
After approx. 1 h, air clusters formed
Table 2. GFRP specimen thickness measurements
#
t [mm]
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1
1.9
2
1.86
3
1.95
4
1.8
5
1.9
6
1.9
7
1.87
8
1.84
Avg
1.88
Stdev
0.05
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NEPORUŠITVENO PREIZKUŠANJE
4-5
2-3
1,59 mm
1,27 mm
1-2
3-4
0,6 mm
0,92 mm
4-5
2-3
1,59 mm
in order to provide the maximum
possible information about the indications of defects in the GFRP
specimen. The C-scan experimental
setup is shown in Figure 5, and the
testing parameters in Table 3.
5-6
0,3 mm
5-6
3-4
1,27 mm
0,92 mm
0,6 mm
0,3 mm
Al-chip
230
4
6
8
 10
 12
 14
Al-foil depth 0,9 mm entire row
All pieces in a row are
between 3rd and 4th layer
Al-foil
Air void clusters
PVC-foil
PVC-foil depth 0,9 mm entire row
260
Avg. thickness: 1.88 mm
Figure 3. GFRP specimen and defect location (rough side of the plate)
in the direction of fibres at various
depths and on both surfaces. The
specimen for the ultrasonic inspection with indicated defects through
the transparent material is shown in
Figure 3.
2.4 Ultrasonic testing
C-scan setup
The ULTRAPAC immersion ultrasonic testing system consists of an
UPK-T24 5-axis motorized immersion tank with 5 motorized axes, con-
trolled and computer composed by
a SMC-8 motion control board and
an IPR-AD1210 ultrasonic acquisition board, Figure 4. Acquisition board includes the high voltage stage
and the signal conditioning and digitalization.
The system is used to perform an
automated ultrasonic inspection
of the GFRP plate by the immersion pulse-echo method. The aim of
this inspection is to optimize the
settings for GFRP plate inspection
In preliminary tests, different transducers were tested on the specimen. To obtain an accurate cartography of the plate in order to
minimize the ultrasonic attenuation
because of the specimen material, a
transducer of 10 Mhz, 3/8”, focused
at 2”, was used.
The water couplant was used to
maintain consistent coupling while
moving and manipulating the transducer which was set to face the
sample surface orthogonally at a
distance of 2” (50 mm) to optimize
the acquired signal. The entire specimen was inspected from both sides, moving the transducer with a
5-axis motorized arm.
To obtain a C-scan cartography of
the specimen, a waveform is acquired on each acquisition point. The
waveform can be seen on the A-scan
cartography. The pulser and receiver
settings given in Table 3 were selected to optimize the waveform shape.
Echoes on the A-scan, shown in
Figure 7, come from the reflecting
surfaces in the material which are
created by the presence of defects
in the sample material.
Figure 4. ULTRAPAC system (on the left); GRPC plate in the immersion tank (on the right)
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NEPORUŠITVENO PREIZKUŠANJE
thickness (Gate 2) and the back wall
echo (Gate 3).
Figure 5. Experimental setup of GRPC plate C-scan (water couplant)
Table 3. C-scan settings
Scanning Axis
Index Axis
Scanning length
[mm]
240
220
Scanning resolution [mm]
0.5
0.5
Scanning
speed [mm/s]
120
120
Pulser
Frequency
10 MHz
Voltage
400 V
Damping
2000 
Receiver
Filter
8-12.5 MHz
Gain
20 dB
On the A-scan, echoes are analyzed
by means of time analysis gates
which provide the maximum amplitude [%] and the associated time of
flight [µs] on a time section of the
waveform.
The A-scan graph in Figure 7 represents the voltage received from the
transducer based on the material thickness where the time is converted into
mm through the calibration method.
The signal values range from the
positive to the negative values of 1
V to –1 V, respectively. Analyzed amplitudes used for C-scan are absolute voltage values expressed in %.
For example, both, –1 V and 1 V are
the 100 % amplitude of the C-scan.
When an echo with a peak at –0.6 V
is detected, the amplitude value
used for the C-scan is 60 %.
When performing the conventional C-scanning, 3 gates are used to
analyze the front wall echo (Gate 1),
echo coming back from the material
In our study, a single gate on the
whole signal from the plate was
used, which allowed us to detect
the presence of defects and their
depth more efficiently. This method
produces 4D C-scan cartographies.
The single gate (Gate 4) was divided
into 14 slices. Each slice corresponds to a narrow depth interval. All slices have equal width (2.174 mm/14
= 0.155 mm). The UTwin software
algorithm locates and records the
signal peak amplitude within the
sliced part of the waveform. Those
two methods are presented in Figures 6 and 7.
The sound velocity in GRPC material was calibrated by measuring the
back wall echo time of flight and the
sample thickness. The time of flight,
which can be measured on the whole signal, equals zero at the beginning of the front wall echo. This calibration gave a velocity of 3000 m/s
for longitudinal waves along the
sample thickness.
■■ 3 Analysis of the results
The 3 following C-scan graphs (Figures 8, 10 and 12) are the result of
the scanning with the transducer on
the smooth side of the composite
plate.
The C-scan in Fig. 8 shows the back
wall echoes amplitude using the
13th slice. The darkest areas descri-
Figure 6. C-scan method with a single gate through the entire signal
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NEPORUŠITVENO PREIZKUŠANJE
Figure 7. A-scan graph with the Analysis Gate (on the left); Gate settings (on the right)
Figure 8. C-scan for 14 slice gate, 13th slice display
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NEPORUŠITVENO PREIZKUŠANJE
same areas as the porosities visible
on the GFRP sample.
The C-scan in Figure 11 shows the
back wall echo amplitude using
the 13th slice (depth: of the large
analysis gate). The darkest areas
describe an amplitude loss of these echoes, which means the signals
have been reflected before reaching
this depth. This C-scan shows the
amplitude of echoes coming from
the depth interval.
The ultrasonic inspection of the
specimens
Figure 12 shows a small analyzed
sample segment with aluminium foils and surface air voids.
Figure 9. Smooth side of the sample
be an amplitude loss of these echoes, which means the signals have
been reflected before reaching this
depth. These indications of defects
are located in the same areas as the
porosities visible on the GFRP plate.
The C-scan on the left in Figure 11
shows the back wall echoes amplitude using the 23rd slice (depth: 1.7741.929 mm) of the 24 slice gate. The
darkest areas describe an amplitude
loss of these echoes, which means
the signals have been reflected before reaching this depth. These indications of defects are located in the
Compared to the framed area on
the photograph of the sample, the
C-scan shows indications of defects
that match with the surface porosity
and a hair located very close to the
smooth surface.
The following three C-scan graphs
(in Figure 13) have been obtained
with the sample’s rough surface facing the transducer.
Figure 10. C-scan for 24 slice gate, 13th slice display
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NEPORUŠITVENO PREIZKUŠANJE
Unlike scanning through the smooth surface, scanning through the
rough face allows us to detect the
PVC foils placed at different depths.
The C-scans above show us indications of the PVC foils nearest to the
rough face.
Figure 11. Detail of C-scan for 24 slice gate, 13th slice display
Only these have been detected in
this area close to the rough face;
the ultrasonic signal is not so attenuated although the front wall echo
has a smaller amplitude (compared
Figure 12. Image of an analyzed detail (Scan resolution of 0.1 mm), slice 2 of 48 slice gate
5th slice of 24 slice gate
6th slice of 24 slice gate
8th slice of 24 slice gate
Figure 13. C-scan for 24 slice gate, 5th slice display
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to the one measured through the
smooth face) because of the surface
roughness which diffracts the signal.
■■ 4 Conclusions
The 4D C-scan method with a divided single gate signal is a powerful tool to obtain C-scan amplitude
data for each depth interval along
the thickness of the whole sample.
For each increment, the algorithm
determines the highest peak of the
signal. The optimal selection of the
number of intervals has to be made
in order to maximize the visibility of
the defect on the C-scans. Also, the
depth of the analyzed defect can
be determined after analyzing all
depth intervals. The depth indication precision can be increased if the
measurement is repeated with classical thickness C-scan, using a gate
placed at the depth range where
the defect indication is visible on 4D
C-scan.
The detection of a defect is limited by its thickness and by the wavelength of the testing ultrasonic
wave. This latter has to be smaller
than the defect thickness to allow
the signal reflection on the defect. Indeed, in our study, the pulse
frequency was set to 10 MHz after
testing optimization. So, as we determined a sound velocity of 3000
m/s, the ultrasonic signal has a wavelength of 0.3 mm.
For that reason, we were not able
to detect the 0.04 mm thick aluminium foil. Nevertheless, by scanning
both sides of the sample, the PVC
foil with 0.1 mm of thickness was
successfully detected in most cases
when the gate was set to 24 divisions. The detection sensibility can be
increased by increasing the testing
frequency, but in this case, the level of noise caused by the sample
structure and the attenuation of the
ultrasonic signal increase drastically.
However, a higher frequency could
be used to estimate very shallow
defects.
The presence of Al-foil can be detected using through-transmission
scanning by analyzing the signal
attenuation. In this case, the defect
size can be obtained on the C-scan,
but not the defect depth.
References
[1] A. Kapadia, Non Destructive Te-
[2]
[3]
[4]
[5]
[6]
sting of Composite Materials, National Composite Network, TWI.
M.R.P. Dasarath Rao, Review of
non-destructive evaluation techniques for FRP composite structural components, Doctoral thesis,
West Virginia, USA, 2007.
R. Grimberg, P. Barsanescu, I
Cortu, S. Leitoiu, A. Savin: Correlation between structures and
phase transition and ultrasound
characteristics at fiber reinforced
plastics composites, 10th International Conference of the Slovenian Society for Non-destructive
Testing, pp. 59-65, 2009.
B. B. Djordjevic: Ultrasonic characterization of advanced composite materials, 10th International Conference of the Slovenian
Society for Non-destructive Testing, pp. 47-57, 2009.
T. Hasiotis, E. Badogiannis, N.G.
Tsouvalis: Application of Ultrasonic C-Scan Techniques for Tracing
Defects in Laminated Composite
Materials, Journal of Mechanical
Engineering, 57(2011), 3, 192203, 2009.
A. Ragondet: Experimental Characterisation of the vacuum infusion process, Doctoral thesis,
University of Nottingham, 2005.
Ultrazvočna preiskava delaminacij in poroznosti v epoxy- kompozitu s steklenimi
vlakni
Razširjeni povzetek
S C-scan pulzno-odbojno ultrazvočno metodo je bila izvedena preiskava epoksidne kompozitne plošče, ojačane s
steklenimi vlakni. Za izdelavo kompozitne plošče smo uporabili postopek vakuumske infuzije za impregnacijo 6 plasti
biaksialne tkanine s keper tkanjem, s težo 280 g/m2 in z enako orientacijo vlaken 0/90° v vseh slojih. Med posameznimi sloji smo na znana mesta vložili napake v obliki okroglih ploščic iz aluminijaste in poliestrske folije in ostružke
aluminija. Za polimerno matrico smo izbrali hladno utrjevalno epoksidno smolo DGBA (diglicidil-eter-bisfenol-A),
Renlam LY 5138, zamreženo s poliaminskim trdilcem Renlam HY 5130 v utežnem mešalnem razmerju 100 : 38. Za
vakuumsko infuzijo smo dimenzionirali dovodne in odvodne kanale ter izvedli infuzijo preko permeabilne mrežice
pri podtlaku –0,9 bar. Med vakuumsko infuzijo smo na dovodni strani povzročili puščanje vakuumske folije, s čimer je
prišlo do nastanka makroporoznosti znotraj kompozitne plošče. Tako izdelana kompozitna plošča ima debelino 1,88
mm, gostoto 1267 kg/m3 in 54-odstotni volumski delež vlaken.
Ultrazvočna preiskava s steklenimi vlakni ojačanih kompozitov težko zazna napake zaradi visoke reflektivnosti in sipanja odbitih ultrazvočnih signalov. Za neporušitveno preiskavo kompozitnega vzorca smo uporabili ultrazvočni sistem
Ultrapac UPK-T24 s 5-osnim manipulatorjem z gibljivo glavo. Med preiskavo sta bila kompozitni vzorec in ultrazvočna
glava potopljena pod vodo za boljši prenos ultrazvočnih signalov do preizkušanca. Na osnovi predhodnih preizkusov
je bila izbrana ultrazvočna sonda (oddajnik-sprejemnik) s frekvenco 10 MHz in z oddaljenostjo 50 mm od površine
preizkušanca. Izvedli smo C-scan z obeh strani po površini z inkrementom 0,5 mm in s hitrostjo 120 mm/s. Z izbrano
metodo smo na vzorcu zajemali ultrazvočne signale le na enem vhodu, ki je bil glede na velikost plošče razdeljen na
14 ravnin. Amplitudne vrednosti signala smo razdelili v posamezne razrede s prikazom v psevdobarvah. Razpozna-
Ventil 20 /2014/ 1
47
NEPORUŠITVENO PREIZKUŠANJE
vanje oblike, velikosti in globine napak je odvisno od valovne dolžine testiranega ultrazvočnega signala. Glede na
izmerjeno hitrost zvoka v kompozitni plošči, ki znaša 3000 m/s, ima ultrazvočni signal valovno dolžino 0,3 mm pri
frekvenci 10 MHz. Z ultrazvokom je možno zanesljivo odkrivati napake, ki so večje od valovne dolžine zvoka. Možna
pa je tudi detekcija napak, ki so nekoliko manjše od valovne dolžine. To se je potrdilo s C-scan kartografijo, s katero
smo na večini mest zaznali poliestrsko folijo debeline 0,1 mm. Če povečamo občutljivost zaznavanja s povečanjem
testne frekvence, se močno poveča šum zaradi številnih odbitih signalov iz kompozitnega materiala. Kljub temu smo
pri višji frekvenci uspešno zaznali tudi zelo drobne podpovršinske napake, kot so npr. nečistoče in pore.
Ključne besede: ultrazvočna preiskava, C-scan, z vlakni ojačani kompoziti, epoksi smola, kompozit, vakuumska infuzija
dr. Zoran Bergant, univ. dipl. inž.
Univerza v Ljubljani, Fakulteta za strojništvo
Acknowledgement
The authors gratefully acknowledge the assistance of Mistras Group, Division EPA.
Znastvene in strokovne prireditve
18. mednarodna konferenca o tesnjenju – Stuttgart 2014 – vabilo k prijavi aktivne
udeležbe
Združenje za fluidno tehniko
v okviru VDMA (Fluid power
Asociation of VDMA) – Skupina
za tesnjenje organizira: 18th ISC
– International Sealing Conference – Internationale Dichtungstagung (18. Mednarodna
konferenca o tesnjenju), ki bo 8.
in 9. oktobra 2014 na Univerzi v
Stuttgartu. Konferenco bo vodil
prof. dr. Werner Haas (Inštitut
za strojne elemente).
Sodobni sistemi tesnjenja so pomembni za delovanje, zanesljivost
in okoljsko kompatibilnost sestavin
fluidne tehnike, kompleksnih strojev
in inštalacij ter vozil. Bistvo delovanja in lastnosti tesnilk in tesnilnih
strojev so mnogokrat skriti pred
uporabniki in kupci.
Zato bo moto konference: Sealing
Technology – Hidden High Tech
(Tehnologija tesnjenja – skrita visoka tehnika).
Namen konference je predstavitev
rezultatov znanstvenih in razvojnih
raziskav, izkušenj uporabnikov ter
izmenjava informacij med vsemi
zainteresiranimi strokovnjaki, ki se
ukvarjajo s tesnjenjem.
48
Prijavite se na spletni naslov: www.
sealingconference.com ali e-pošto:
christian.geis@vdma.org.
Povzetke prispevkov v nemščini ali
angleščini pošljite najpozneje do 28.
februarja 2014 na naslov kontaktne
osebe.
Tematike 18. ISC bodo naslednje:
yy statične tesnilke
yy gredne tesnilke
yy sem-in-tjakajšnje tesnilke (hidravlika, nevmatika)
yy osnove tehnologije tesnjenja
yy materiali in površine
yy ohranjanje energije, trenje, obraba
yy simulacije
yy standardizacija, predpisi, postopki preskušanja
yy uporaba in praksa
Vzporedno s konferenco bo organizirana priložnostna razstava.
Kontaktna oseba:
Fachverband Fluidtechnik im VDMA
18th ISC
Dr. Christian Geis
Postfach 71 08 64
60498 Frankfurt/Main, Germany
tel.: +49 (0) 69 66 09-1318
faks: +49 (0) 69 66 09-2318
e-pošta: christian.geis@vdma.org
Vir: Geis. Ch.: 18th ISC – International
Sealing Conference Stuttgart 2014 –
VDMA – Fluidtechnik – Press Release, 2013-11-20
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